High voltage transmission line parameters include resistance, reactance, and equivalent admittance which represent reactive charging power along the line. These parameters are a necessary and important input in power system modeling, power flow computations, voltage stability assessment, line protection design and other applications. In the prior art, the transmission line parameters are calculated using theoretically derived formulas based on information of the line's size, length, structure and type, etc., which are assumed to be constant during the power flow modeling. However, there is a difference between the calculated and actual parameters. Some prior art methods have been developed to measure the resistance and reactance of a line. However, the equivalent admittance representing reactive charging power is not measurable. Also, a “snapshot” off-line measurement of resistance and reactance parameters is not sufficient for on-line and real time applications, because in a real live environment, resistance, reactance and equivalent admittance of lines vary with environment and weather (such as temperature and wind speed). Therefore, the assumption of constant parameters may create unacceptable error, particularly when the environment or weather around the line has a relatively large change.
U.S. Pat. No. 5,631,569, entitled “Impedance measurement in a high-voltage power system” discloses a power monitoring instrument for evaluating and displaying the source impedance, load impedance, and distribution system impedance. The focus of this patent is placed on sources, loads and distribution systems but not on transmission line parameters. U.S. Pat. No. 5,818,245, entitled “Impedance measuring”, discloses a measurement method for impedance of a power system adapted for operation at a predetermined line frequency. This patent focuses on the effect of frequency on the measurement and requires a testing signal of frequency. U.S. Pat. No. 6,397,156, entitled “Impedance measurement system for power system transmission lines” discloses an impedance measurement method to improve various protection functions. All of the above methods are not designed for an application operating in real time or for on-line power flow modeling and calculations, and also cannot provide an estimation of equivalent admittance representing reactive charging power of a transmission line.
PMU technology has developed quickly in the utility industry of both developed and developing countries in recent years since the basic concept was presented, as described in A. G. Phadke, “Synchronized Phasor Measurements in Power Systems”, IEEE Computer Applications in Power, April 1993, pp 10-15; M. Zima, M. Larsson, P. Korba, C. Rehtanz and G. Anderson, “Design aspects for wide-area monitoring and control systems”, Proceedings of IEEE, Vol. 93, No. 5, May, 2005, pp 980-996; “Eastern Interconnection Phasor Project”, 2006 IEEE PES Power Systems Conference and Exposition, 2006 (PSCE '06), Oct. 29-Nov. 1, 2006, pp 336-342; and Xiaorong Xie, Yaozhong Xin, Jinyu Xiao, Jingto Wu and Yingduo Han, “WAMS applications in Chinese power systems,” IEEE Power & Energy magazine, Vol. 4, No. 1, January/February 2006 pp 54-63. The application of PMU is currently limited to phasor monitoring, enhancement of system state estimator and protection relays as disclosed in U.S. Pat. No. 684,533, entitled “Protective relay with synchronized phasor measurement capability for use in electric power systems” and U.S. Pat. No. 7,069,159, entitled “Electric power transmission network state estimation”.